The present invention relates generally to a sensor for detecting vibrational disturbances, and more particularly to a fiber optic, three-dimensional, sensor capable of detecting and measuring disturbances of the sensor as caused by motion or vibration.
Various alarm systems sensitive to motion or vibrational disturbances are presently used for protecting homes, autos, businesses, machinery, and other valuables from intrusion or theft by providing appropriate signals when the alarm system senses an intrusion into the sensor-protected area or a disturbance to a sensor-protected object. The more sophisticated alarm systems are capable of detecting movements or vibrations over a relatively wide range of frequencies and amplitudes. Also, other systems rely upon the measurement of mechanical vibrations for assessing the condition of machines and mechanical equipment. Many of these wide band alarm and sensor systems have been found to be relatively complex and commonly rely on servo accelerometers and their accompanying electronic modules for providing the desired response to motion or vibrational disturbances over a selected range of frequencies and amplitudes.
More recent developments in intrusion detecting systems utilize an arrangement of optical fibers sensitive to the weight of intruder for providing the alarm. One such intrusion detector system is described in U.S. Pat. No. 4,482,890 where light pulses are transmitted through one of two parallel arrays of optical fibers. When light transmitting the optical fiber array is subjected to "micro-bending" by the weight of the intruder, this fiber array undergoes deformation so as to cause the light pulses passing through the fiber array to be refracted through the light-reflecting cladding of the optical fiber into the other optical fiber array to provide light pulses therein which provide a signal indicative of the intrusion. Another fiber optic sensor development utilizes single mode optical fibers which are arranged to interferometrically sense motion. One such interferometric fiber optic sensor is a Michelson interferometer in which light from a laser is split into two beams by a splitter/coupler for transmitting a beam of light through each of a pair of single mode optical fibers. One of these two optical fibers provides a reference signal while the other optical fiber provides for motion sensing signal. The two laser light beams transmitted through the optical fibers are reflected at the ends thereof and recombined at the splitter/coupler and then transmitted to a photo detector. Any optical path differences that occur in the light transmitted through the motion sensing optical fiber produces a change in the output signal from the photo detector with this change being indicative of the vibration or motion introduced into the motion sensing optical fiber of the interferometer. A Michelson Interferometer is generally described in the publication "Introduction to Optical Fiber Components and Systems" by Michael Corke, Optical/Optoelectronic Engineering Update Series, Update Course U5, Winter 1987, pp 63-65 and 104-119. This publication is incorporated herein by reference.
In the presently known interferometers using single mode optical fibers such as in the Michelson Interferometers, both of the optical fibers are very sensitive to vibration and/or motion. Thus, since the optical fiber providing the reference signal is subjected to vibrations and motion, especially during abrupt movement or when subjected to high frequencies or amplitudes, the output signal from the interferometer can be compromised so as to provide an output signal that often inaccurately reflects the extent of the displacement of the motion sensing optical fiber as caused by the intrusion of the alarmed area or the movement of the alarmed object. This sensitivity of the optical fiber providing the reference signal detracts from the utilization of the sensor as a motion or vibration sensor especially one capable of accurately measuring the extent of the intrusion or motion.